Crown rust of oats is the most common leaf disease of oats in the Western Cape, South Africa. It is caused by a fungus named Puccinia coronata f. sp. avenae and can cause major damage in susceptible cultivars. Although the word “crown” is referred to in the name, which may be confusing, it is basically a leaf disease. The biology of the disease is like that of other rust diseases of small grains, so-called because the colour of the symptoms is thought to be like that of rusting iron.
The fungus is a parasite and survives only on living plants – and especially wild oats, which is a common weed in many small grain production systems. The spores, which are like seeds of plants, but just the reproductive structures of fungi, are carried into the air and land on the healthy tissues of susceptible oat plants. Under favourable conditions, the spores germinate (again very similar to the way seeds do) and infection takes place.
The symptoms that become visible on the leaf about two weeks after infection are bright orange, oblong oval pustules. The word pustules are used because it literally breaks out like small pustules that we would see on human skin. The symptoms carry thousands upon thousands of new spores that can re-infect nearby leaves and plants. These pustules occur mainly on the leaves but can also occur on the leaf sheaths and glumes.
The green leaf area of the leaf gives the plant its production capacity. Green leaf area is important for photosynthesis, the process by which sunlight energy is captured by plants and converted to carbohydrates. As the fungus diminishes the green leaf area, the production capacity of the plant also diminishes. The loss in yield is usually related to the level of infection experienced in the leaves. The quality of the oat grain can also be severely hampered. Under high infection pressure, this disease can cause up to 40% yield loss. About a third of the oat cultivars planted nationally in South Africa are susceptible to crown rust.
The disease appears around mid-July until the end of August. The application of fungicides around the flag leaf stage will protect the leaves of the plant from the disease. Pallinup, SSH421 and SSH405 are susceptible to crown rust and producers should look specifically out on these cultivars for the disease.
Planting resistant cultivars remain the best option for controlling the disease. Resistant cultivars will also not require fungicide application or can be monitored for the presence of other diseases before treating with fungicide. Resistant cultivars are SSH4185, SSH 4186 Magnifico, Wizzard, Basaia and Saia.
The complete susceptibility status of oat cultivars can be found in the small grain production manual published by the Agricultural Research Council Small Grain Institute. The manual is available online (open production guideline tab at top of page).
Contact the author Dr Ida Wilson at firstname.lastname@example.org
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